View source for User Reference:Timing

==Measuring Source-to-Stimulus Delay==
In the ''StimulusTime'' state, the application module stores a time stamp when the module's Process() function has finished. By this time, video memory should have been updated, given that the module enforces a display update at the end of its Process() function; generally, this is done by calling the <tt>[[Programming Reference:GraphDisplay Class#Update|GUI::GraphDisplay::Update()]]</tt> function for the module's display window. Modules that use the [[Programming Reference:ApplicationWindow Class|ApplicationWindow]] class for stimulus display (as most do), don't need to call <tt>GUI::GraphDisplay::Update()</tt>, as this is taken care for by the ApplicationWindow class after all calls to <tt>GenericFilter::Process()</tt> have returned.

Once the stimulus time stamp has reached the source module, the [[User Reference:DataIOFilter|DataIOFilter]] subtracts the source time stamp from the stimulus time stamp to compute the source-to-stimulus delay. Unlike roundtrip time, measurement of the source-to-stimulus delay requires that data acquisition and application modules share a common time base; when BCI2000 is distributed over multiple machines in a network, source-to-stimulus delay cannot be measured.